A rotor assembly for a vehicle alternator generally comprises the following: a rotor shaft, a slip ring assembly, a pair of pole pieces which conduct magnetic flux, a coil wire held between the pole pieces, and a pair of stamped steel cooling fans welded to respective outboard faces of the pole pieces. The rotation of the cooling fans creates air circulation through the alternator to cool the alternator. However, use of these stamped steel cooling fans has several shortcomings.
First, the number and size of blades formed in the stamping operation are limited by the finite amount of sheet steel which is stamped to form the fan. This finite amount or volume leads to a trade off between blade height, length and total number of blades. Accordingly, the total surface area of blades on a stamped metal fan is restricted.
Second, configurations of blade geometries on stamped steel fan blades are limited. The blade geometries must be compatible with matching dies forming the blades. Therefore, stamped blade configurations are limited as compared to those attainable through other modes of construction.
Third, edges of stamped fan blades are usually sharp and somewhat rough due to the shearing operation used to cut the fans from the metal sheet. The sharp and rough edges disturb the airflow in an alternator as air passes by the blade edges. Further, the jagged edges may cause cuts and scrapes on workers handling the stamped fans.
Fourth, another concern in high-speed alternator design is the amount of imbalance existing in a rotor assembly. When stamped metal fans are welded to pole pieces, the fans typically introduce a significant amount of imbalance to the rotor assembly. The greater the imbalance, the greater the work required to bring the rotor assembly into balance. Also, many rotor assemblies use varnish to hold components of the rotor assembly in place. This may add to the imbalance of the rotor assembly.
Finally, using stamped metal fans with a rotor assembly adds several operations to the construction of the rotor assembly. The fans must first be stamped from sheet steel. Second, the fans must be mounted and welded to the pole pieces. Third, imbalances associated with the addition of the stamped metal fans must be corrected through an iterative balancing operation.
The present invention addresses problems associated with the above-identified shortcomings.